ECN Forum Forums Code Related Discussion NEC & other Code issues SERVICE ENTRANCE CONDUCTORS

what size service entrance conductors are reqired for a 400 Amp 120/208Volt 3phase 4wire service, with a 400 Amp Main breaker?
The engineer says 600kcm cu, I say 500kcm cu
who is correct?

Exception #2 to 230.90(A) would permit the use of 500 kcmil copper, however the designer, if he chooses to, can require conductors larger than the NEC mimimum.
Don

Small aside: "Code" 600kcmil is easier to work than 500, for it is 61-strand compared to 500's 37-strand.

Who's buying you or them.

Make the engineer pull it......

RSlater,
RSmike

Don,
If the engineer is specifying a 400 amps service wouldn't 600's be required? With 500's you would only have a 380 amp service. I realize this is being picky and I'm sure the load on this new service is not that close. What happens in a few years when some additional equipment is added and the service load is calculated at 390 amps? If the original contract with the property owner called for a 400 amps service then the engineer or electrical contractor is going to coming back at their cost and replacing the 500's.

Curt

Curt,
Given the normally accepted 80% rule for overcurrent protective devices, the 400 amp OCPD should only have a 360 amp load. If the calculated load is 390, a 400 amp service is not big enough. The code does require that the service conductors be sized for the load and if the load exceeds 380 amps, then the 500 kcmil are too small.
Don

Don,
You are correct for my example to work the 390 amps would have to be after 125% is applied to all continuous loads. I just try to be careful before reducing the size of wire. If plans specifically call for a 400, 600 or 800 amp service I want to make sure all parties involved approve before I give them slightly less. I don't want worry about any future problems. In reality even if the load on this service did increase to 400 amps the transformer and conductors on the utility side will probably fail long before the customer installed 500's.

Curt

Why don't you run parallel 3-Ø in 2 - 2" conduits?

Don,
The reason we have personal differences on this issue is the fact that we do not know how the 400 A service was computed.

I think the 400A already accounted for the non-continuous/continuous character of the load( ~400A = NCont + Cont) therefore the size of SE should be 600KCM per 215.2(A) & 215.2(3) . Likewise a 400 A breaker should be selected per 215.3.

Glenn

Glenn,
You are correct, the load calcs are the key to this question. If the calculated load is 380 or less, the 500 kcmil is fine. If the load is over 380 but equal or less than 400, you will need 600 kcmil. If the load calc exceeds 380 amps, we really should be looking at a larger service. In this area the generally accepted wire for a 400 amp service is 500 kcmil.
If the engineer has specified 600 kcmil, that is what should be installed even if the load calc is is less that 380 amps. The engineer is permitted to exceed the code minimum.
Don